Abstract

The convergent gliding adaptations in four mammalian families (Sciuridae, Anomaluridae, Cynocephalidae, and Phalangeridae) are compared. These adaptations are contrasted with the non-gliding arboreal genera Sciurus, Pseudocheirus, and Hemibelideus. Both functional and evolutionary aspects of the problem are presented. The gliding membranes with their associated musculature and cartilages and the musculature and bones of the pectoral appendages were dissected in the following fluid-preserved genera: Glaucomys, Pteryomys, Iomys, Petinomys, Petaurista, Anomalurus, Cynocephalus, Schoinobates, Petaurus, and Acrobates. Descriptions and figures of these anatomical structures are presented. Measurements of the forelimb and its parts were taken and are presented primarily as ratios.^ Despite the tendency in recent years to split the Petauristinae into a number of groups on the basis of evidence from teeth, bacula, ear region, and immunology, evidence from the musculature of the patagia and pectoral limb, the osteology of the pectoral limb and the presence in all genera of a styliform cartilage with nearly the same structure suggests that this group may be monophyletic. The petauristines share 72(67%) osteological characters out of 109 studied that are different from Sciurus. All gliders share only 23% of the same characters. Within the petauristines studied, various stages of primitive (Glaucomys) versus derived (Petaurista) characters are elucidated. Osteological characters which may prove useful in determining whether or not a fossil sciurid is a glider are listed.^ The anatomy of the gliding phalangers and that of Pseudocheirus shows that all of the gliders could have evolved from a Pseudocheirus-type ancestor. The anatomy of Hemibelideus suggests that it should be considered as a separate genus rather than as a subgenus of Pseudocheirus. Schoinobates is convergent with the rodent genus Anomalurus in the presence of a styliform cartilage which articulates with the olecranon process. Osteological characters which may prove useful in determining whether or not a fossil phalanger is a glider are listed.^ Special functional adaptations found in the pectoral limb of the gliding genera are: increased size of the M. supraspinatus providing increased force of outward rotation of the humerus at the shoulder joint, increased length of insertion of the pectoralis major providing a brace for the forelimb against the force of air hitting the ventral surface of the patagia, the metacromion of the scapula and the greater tubercle of the humerus act as a locking mechanism in the shoulder joint, and the coracoid process of the scapula and the muscles of the rotator cuff brace the shoulder joint.^ The patagia of the gliders are edged with rope-like muscles which help maintain the shape of the membranes by isometric contraction. While not gliding, relaxation of these muscles and contraction of the humerodorsales I and III and of the flexor carpi ulnaris (Petauristinae) helps to pull the membranes and styliform cartilage toward the body. No mechanism for pulling the styliform cartilage of Anomalurus toward the body could be elucidated. During the glide, the humeroventralis, humerodorsales, and the transverse muscles are probably used to control the amount of upward billowing in the membrane caused by the force of the air and allow the animal to change its angle of attack during the glide.^ The bones and muscles of the pectoral limb are more robust in the non-gliders indicating they climb up and down trees more frequently than do the gliders. The osteological and myological characters of Hemibelideus indicate that the members of this genus are probably capable of limited gliding. ^